Could not load file or assembly 'System.Web.Http.WebHost, Version=5.2.3.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies

I have purchased the course: ”Sitecore® 8.2 Experience Solution Developer Course” but have an issues that prevents me from progressing. The error I see is this:

Configuration Error

Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: Could not load file or assembly 'System.Web.Http.WebHost, Version=5.2.3.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Assembly Load Trace: The following information can be helpful to determine why the assembly 'System.Web.Http.WebHost, Version=5.2.3.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' could not be loaded.

Jan is correct... This error is very common when the offending DLL that is in your "bin" folder is a different version than is specified in your web.config. Most of the time, this happens because the referenced NuGet DLL in your source code is a different version than what Sitecore shipped with and when you publish your solution, the mismatched DLL gets deployed to the "bin" folder. Alternatively, it could also be because the web.config file was modified with a version that is not what Sitecore shipped with. In any case, the correct resolution would be to use the DLL version that your Sitecore version initially comes with. Adjust either your web.config, or the NuGet package that you are referencing so that the DLL versions are matching.